The present invention relates to a multiband frequency generation with a PLL-circuit. In particular, the present invention relates to a multiband frequency generator to generate an output signal in a plurality of frequency bands and, further to a sending/receiving unit wherein the multiband frequency generator according to the present invention may be used.
FIG. 8 shows a typical method for sending and receiving signals in mobile phones. In the receiving path, a first mixer stage 204 comprising a multiplier 200 and a band pass filter 202 is supplied with a local oscillator signal outputted through a frequency generator 206 such that at the output of the mixer stage 204 the receiving signal is available according to a fixed intermediate frequency (IF) for the further processing in downstream circuit unit (not shown).
As also shown in FIG. 8, when sending a modulated sending signal (provided in the base-band) this sending signal is converted to an intermediate frequency-band specified through a sending intermediate frequency signal using a second mixer stage 208 with a second multiplier 210 and a second band-pass 212. Then, the conversion to the sending channel is executed through a third mixer stage 218 comprising a third multiplier 214 and a third band-pass filter 216.
FIG. 9 shows a detailed schematic diagram for the frequency generation unit 206. Here, the object is to tune the frequency of a voltage-controlled oscillator 220 such that it is coincident to a frequency of a basic oscillator 222 multiplied with a dividing factor of a second programmer divider 228. In FIG. 9, specific numerical values are given for a GSM-application example in square brackets.
The basic oscillator 222 comprises a reference oscillator 224 and in addition a first programmable divider 226 to variably pre-specify a reference frequency. A second programmable divider 228 is provided to convert the frequency generated by the voltage-controlled oscillator 222 into the pre-specified reference frequency outputted by the basic oscillator 222.
A phase detector 230 enables a comparison of the sending signal converted with the second programmable divider 228 and the reference signal outputted through the first programmable divider 226. The phase error between the divided reference signal and the divided output signal of the voltage-controlled oscillator 220 determined by the phase detector 230 is supplied to a loop filter 232 where an integration takes place.
Using this integrated error signal, the voltage-controlled oscillator 220 is controlled until there exists no further frequency and phase difference, respectively, between the signals used for comparison. Herethrough, the voltage-controlled oscillator 220 has a relative stability that is identical to the relative stability of the basic oscillator 222. For the example shown in FIG. 9, e.g., in case the relative stability of the basic oscillator is (1 Hz)/(200 kHz) the relative stability for the voltage-controlled oscillator 220 is (3860 Hz)/(772 Mhz).
For applications such as GSM 900, GSM 1800 or PCS-mobile telephony, the tuning of the receiving or sending channel is carried out through determination of the divider ratio for the first and second programmable divider 226 and 228, respectively. Therefore, the voltage-controlled oscillator 220 may easily be tuned to different sending frequencies within a stable operation. Here, the adjustment behaviour and the stability is essentially determined through the design of the loop filter 232.
The design shown in FIG. 9 is suitable for mobile phones being operated in one frequency band. However, this single-band operation is no longer suitable in view of the increasing number of subscribers and the limited number of sending frequencies in existing cellular mobile networks.
To the contrary, a combination of technical advantages being related to different approaches seems to be promising, in particular through providing multiband cellular networks and multiband mobile phones being related thereto, e.g., through combining the GSM 900, GSM 1800 and PCS frequency bands.
However, a prerequisite to this approach is the frequency generation for the respective frequency bands while simultaneously meeting the strict requirements explained with respect to FIGS. 8 and 9. For a dual band operation two frequency generators will be required for the two frequency bands. However, it is not possible to use only a single voltage-controlled oscillator since the tuning range that will be necessary in case of a single voltage-controlled oscillator would lead to an influence of noise onto the system that is too large and therefore to a violation of pre-defined specifications. Therefore, two voltage-controlled oscillators operating independently should be provided for the dual band operation mode.
The direct generalization of the approach explained with reference to FIG. 9 would be a duplication of the components according to the number of pre-specified frequency bands. While this allows to provide frequency signals in an independent manner through related phase-locked loop circuits, this may only be achieved with significant additional circuitry and additional costs. Further, increased space requirements constitute a barrier against this approach, since a plurality of frequency synthesis units are necessary, e.g., in the form of a plurality of integrated circuits.
In view of the above, the object of the present invention is to provide a multiband frequency generator with minimized circuitry requirements.
According to the present invention, this object is achieved through a multiband frequency generator to generate an output signal in at least two frequency bands, comprising a voltage-controlled multiband oscillator to generate an output signal in each frequency band at one output for each frequency band, a frequency synthesis means to derive a phase difference between a control input signal for the frequency and the generated output signal, at least one control means for the voltage-controlled multiband oscillator using the phase difference generated through said frequency synthesis means as correcting or manipulating variable, respectively, wherein each output terminal of said multiband oscillator is coupled to the frequency synthesis means via a frequency selective coupling means.
Therefore, according to the present invention, only a single frequency synthesis means must be used since different output branches of the multiband frequency generator are always coupled to the same frequency synthesis means in a frequency selective way. The frequency selective behaviour of the coupling unit enables an excellent decoupling of the different single oscillator units of the voltage-controlled multiband oscillator.
Further, the invention enables a low loss between the different oscillator units of the voltage-controlled multiband oscillator and the phase-locked loop circuit comprising the frequency synthesis means and the control means.
Still further, the frequency selective coupling means also enables an impedance matching between the oscillator units of the voltage-controlled multiband oscillator and the phase-locked loop circuit and in addition a DC-decoupling.
According to a further preferred embodiment of the present invention, a control means is provided for each frequency band.
The provision of a single control means for each frequency band enables the individual determination of the adjustment behaviour and the stability of the different voltage-controlled oscillators.
According to a further preferred embodiment of the present invention, the voltage-controlled multiband oscillator comprises a plurality of voltage-controlled single band oscillators connected in parallel. Alternatively, there may be provided a switchable voltage-controlled oscillator. Both approaches enable the flexible handling of pre-defined specifications in that the number of voltage-controlled single band oscillators and switching stages of the switchable voltage-controlled oscillator, respectively, is adjusted to the number of frequency bands that must be generated.
According to a further preferred embodiment, the frequency selective coupling means comprises a filter bank.
Hereby the decoupling and impedance matching may be optimized in a scalable manner in dependence of the number of pre-specified frequency bands and also in dependence of the specifications to be fulfilled.
Overall, the present invention enables the multiband-frequency generation with only a single PLL-circuit while using only a single frequency synthesis means through switching the supply voltage between the oscillator units of the voltage-controlled multiband oscillator and also through frequency selective coupling of the output signal at the phase-locked loop circuit.